Pressure transducer with reduced temperature sensitivity

ABSTRACT

Disclosed is a piezoelectric pressure transducer particularly suited for use with an engine gauge for measuring pressures in internal combustion engines or for sensing pressure in other severe temperature environments. A thin, flat diaphragm is protected by a ceramic shield and other elements of the gauge are made of material having a low thermal expansion such as invar. Access to the diaphragm is by way of a small annular, stepped groove to cool the gases acting on the diaphragm. A piezoelectric element such as quartz having two sensitive axes is employed and long term temperature effects are nullified by applying compensating stresses to the second axis of the piezoelectric element.

[ 1 June 27, 1972 [54] PRESSURE TRANSDUCER WITH REDUCED TEMPERATURESENSITIVITY [72] Inventor: Vernon H. Siegel, Clarence, N.Y.

Sundstrand Data Control Incorporated, Redmond, Wash.

22 Filed: Dec. 3l, 1970 [21] Appl.No.: 103,330

[73] Assignee:

Related US. Application Data [63] Continuation of Ser. No. 22,132, Aprill, 1970, abandoned, which is a continuation of Ser. No. 703,621, Feb. 7,1968, abandoned,

3,349,259 lO/l967 Kistler ..3l0/8.7

3,364,368 l/l968 Sonderegger ..310/8.7 3,390,287 6/1968 Sonderegger ..3l0/8.7 3,393,331 7/1968 Puckett A .3 l0/8.7 X 3,424,930 l/l969 List eta1. l0/8.7 3,461,327 8/1969 Zeiringer .3 l0/8.7 X 3,482,122 12/1969Lenahan ..310/9.l 3,497,728 2/1970 Ostrofsky ..310/9. 1 X

Primary Examiner-L. T. Hix Assistant Examiner-Mark O. Budd AttorneyLeBlanc & Shur [57] ABSTRACT Disclosed is a piezoelectric pressuretransducer particularly suited for use with an engine gauge formeasuring pressures in internal combustion engines or for sensingpressure in other severe temperature environments. A thin, flatdiaphragm is protected by a ceramic shield and other elements of thegauge are made of material having a low thermal expansion such as invar.Access to the diaphragm is by way of a small annular, stepped groove tocool the gases acting on the diaphragm. A piezoelectric element such asquartz having two sensitive axes is employed and long termtemperatureeffects are nullified by applying compensating stresses tothe second axis of the piezoelectric element.

2 Claims, 16 Drawing Figures PATENIEDJum I972 SHLET 2 OF 3 1 INVENTORVERNON H. SIEGEL ATTORNEYS SENSITIVITY, which is in turn a continuationof my then copending application Ser. No. 703,621, filed Feb. 7, 1968,now abandoned.

This invention relates to piezoelectric pressure transducers and moreparticularly to a transducer suited for use in measuring pressurechanges accompanied by'extreme temperatures such as are encountered incylinders of internal combustion engines and in explosion testing. Thetransducer is constructed to minimize transducer output due both toshort duration high temperature flashes as well as long duration mediumtemperature variations.

In general, piezoelectric crystals produce a charge output in responseto a stress generated as a result of a force applied to the crystal.Certain ferroelectric crystals such as barium titanate also produce acharge when the crystal is exposed to a change in temperature. Othercrystals, such as quartz, do not exhibit an output due to temperaturewhenthe crystal is free, but when clamped between materials having adifferent coefficient of expansion, an output charge may result fromquartz with any change in temperature. Also, in a practical transducerhaving' a flat diaphragm, a charge output can be produced by flexing ofthe diaphragm due to a temperature gradient across the thickness of thediaphragm. Additional undesirable o'utputs maybe generated intransducers as a result of expansion 'of a prestressing sleeve or otherstructures that are frequently used to clamp the piezoelectric crystals.

Because of these temperature effects, accurate measurement of dynamiccylinder pressure of an internal combustion engine is quite difficult,particularly measurement of the pressures during the scavenging andintake portions of the engine stroke where the pressure is of the orderof psi and occurs within a few milliseconds after a pressure of 1,000psi or higher which higher pressure is accompanied by flashtemperatures'of 3,000 F. or more.

Transducers have been constructed where the sensing element is connectedthrough a narrow passage to the cylinder chamber in order to minimizetemperature effects. In general, these are unsatisfactory, since theresonance of thepassage produces ripples in the pressure diagram.Additionally, if the sensing element does produce an output fromtemperature, the heat generated from the compression of the gases in thepassage is generally {sufficint to produce distortion of the pressureoutput. I v v These and other problems are overcome by the pressuretransducer of the present invention, particularly suited for use with anengine pressure gauge.' In particular, the present invention provides anarrangement for isolating the piezoelectric element of the transducerfrom temperatures so that short duration, high flash temperaturesin theorder of 3,000 F. or more have little or no efiect on the output. Alsoincorporated in the unit are elements having different coefficients ofexpansion for long term temperature compensation especially suited foruse with piezoelectric materials such as quartz and the like which havemore than one electrically sensitive axis. The improved transducer ofthis invention has good high frequency response, is easier to work andmanufacture and offers ap proximately a 10-fold increase in accuracy ascompared to existing devices for measuring internal combustion enginepressures. It is capable of measuring pressuresas high as 1,000 psi andhigher and as low as A psi in extremely severe temperature environments.

In the transducer, a prestressed piezoelectric package is acted upon inresponse to pressures by a flat diaphragm. The diaphragm is to a largeextent isolated from short term high temperature shocks by a ceramicblock or heat shield having ,both a low coefficient of thermal expansionand a low thermal conductivity. Cooperating with the heat shield is aheat shroud which defines a relatively narrow and deep annular groove tofurther reduce the temperature of the pressure gases acting on thediaphragm. A novel heat shield retainer provides a stepped area at theinner end of the groove to further enhance the cooling effect of thegases. In this way, the outer end of the groove may be suflicientlylarge such that in conjunction with the ceramic material forming theheat shield, it assures rapid bum-ofl of any carbon deposits which mightotherwise tend to collect and adversely interfere with the operation ofthe transducer.

In addition, long-term temperature compensation is provided in thedevice of this invention by matching the coefficient of thermalexpansion of various elements of the device so as to exert anappropriate stress along a second active axis of the piezoelectricmaterial which, by way of example, may

be an X-cut quartz crystal having two mutually perpendicular electricalaxes in addition to the third neutral or optical axis. Thus, long-termtemperature effects which might otherwise produce an undesirable outputdue to stresses along the first axis are counteracted. and nullifiedthrough the fact that this same long-term temperature build-up producesstresses on the crystals along the second axis to nullify the long-termeffect.

It is therefore one object of the present invention to provide animproved temperature insensitive piezoelectric transducer.

Another object of the present invention is to provide a transducerparticularly suited for measurements of pressure changes accompanied byextreme temperatures.

Another object of the present invention is to provide a pressure gaugeparticularly suited for use in measuring pressures encountered ininternal combustion engines and in explosions.

Another object of the present invention is to provide a piezoelectrictransducer having both short-term temperature immunity and long-termtemperature compensation which is particularly useful in conjunctionwith gauges for measuring the pressures occurring in internal combustionengines, particularly the pressures occurring in the engine during thescavenging and intake portions of the stroke as well as the higherpreceding engine pressures.

These and further objects and advantages of the invention will be moreapparent upon reference to the following specification, claims andappended drawings wherein:

FIG. I is a vertical elevation with parts in section showing the novelpressure transducer of the present invention;

FIG. 1A is an enlarged view of the left-hand end of the transducer ofFIG. 1;

FIG. 2 is a similar vertical view with parts in section showing theprestressed quartz subassembly forming partof the transducer of FIG. 1;

FIG. 3 is an end view of an end piece for the subassembly of FIG. 2;

FIG. 4 is a side view of the end piece of FIG. 3;

FIG. 5 is an end view of the heat shield retainer forming a 7 part ofthe transducer of FIG. 1;

FIG. 6 is a sectional view of the heat shield retainer of FIG.

FIG. 7 is an end view of the ceramic heat shield incorporated in thetransducer of FIG. 1;

FIG. 8 is a side view of the heat shield of FIG. 7;

FIG. 9 is an end view of the heat shroud completing the transducer ofFIG. 1;

FIG. 10 is a sectional view of the heat shroud of FIG. 9;

FIG. I1 is an elevational view of an acceleration compensated quartzcrystal package usable in the pressure transducer of FIG. 1;

FIG. 12 is an end view of the quartz package shown in FIG.

FIG. 13 is an end view of the seismic mass forming a part of the crystalpackage of FIGS. 1 1 and 12;

FIG. 14 is a side view of the seismic mass of FIG. 13; and

FIG. 15 is an end view of one of the gold electrodes incorporated in thequartz crystal package of FIGS. 1 1 and 12.

Referring to the drawings and particularly to FIGS. 1 and 2, the novelpiezoelectric pressure transducer of the present invention is generallyillustrated at 10 in FIG. 1. The transducer comprises a conductivemetallic base 12 provided with a cavity 14 at its rear end frictionallyreceiving a coaxial connector 16. Connector 16 is preferably providedwith an outer flange 18 are welded to the rear end of the base 12.Connector 16 is provided with a stud 20 at its forward end. The stud iscrimped to one end of a lead wire 22 constituting the active output leadfrom the transducer. The other side of the transducer output is by wayof the grounded base 12 and this base is provided with a centralaperture 24 into which stud 20 projects and which aperture also receivesa hollow circular insulator 26 electrically isolating the two sides ofthe transducer output.

Enlarged aperture 24 centrally located in base. 12 communicates with asmaller aperture 28 which receives a second hollow cylindrical insulator30 surrounding output lead 22. Insulators 26 and 30 are preferably madeof dense alumina (98 percent A1 having a volume resistivity at 300 C. ofOhms. Both insulators are joined to each other and to the base 12 by aninsulating adhesive such as epoxy as illustrated at 32 in FIG. 2.

Joined to the base 12 over the reduced diameter shoulder 34 is ametallic case 36 preferably made of invar having a thermal coefficientof expansion of less than 1 part per million per degree centigrade. Case36 is preferably joined to base 12 by arc welding at the joint 38 inFIG. 1. Also joined to the base and surrounded by casing 36 is aprestressing sleeve 40 preferably also made of invar and enclosing aquartz wafer package generally indicated at 42 which is sandwichedbetween the outer end of base 12 and a flat solid cylindricaltemperature compensating plate 44. Completing the subassembly of FIG. 2is a flat solid cylindrical bufi'er plate 46, a ceramic piece or body 48also of solid cylindrical configuration, and an end piece 50.Prestressing sleeve 40 is open at both ends but includes an inwardlyturned annular flange 52 overlying end piece 50 at its one end and anoutwardly turned or thickened annular flange 54 at its other end. Sleeve40 may be stretched by applying a suitable tool to thickened or flangedend 54 to prestress the quartz crystal wafer package 42 by squeezing itbetween compensating plate 44 and the outer end of base 12. Sleeve 40 ispreferably spot welded by a plurality of circumferentially spaced spotwelds to buffer plate 46 as illustrated at 58 and to the base 12 at twolocations as indicated by the annularly spaced series of spot weldsreferenced at 60 and 62. Each set of annularly spaced spot welds 56, 58,60, and 62 are preferably 12 in number and are spaced at equal anglesabout the circumference of prestressing sleeve 40. These welds arepreferably formed in an alternating sequence to minimize stresses in themanner shown in FIG. 3 ofU.S. Pat. No. 3,35 1,787, issued Nov. 7, I967.

Spot welded to the end of case 36 is a completely flat circular metaldiaphragm 64. Diaphragm 64 is preferably welded to the end of the casewith a continuous weld 66 on a 0.212 inch diameter. In the preferredembodiment, the diaphragm has an overall diameter of 0.234 inches and isformed of annealed invar having a thickness of 0.003 inch. In turn, spotwelded to the diaphragm, as at 68, is a heat shield retainer 70 ofgenerally cup-shaped configuration having its outer edge rolled over asat 72 into a groove 74 in a ceramic heat shield 76. Surrounding ceramicheat shield 76 and spaced from it by annular groove 78 is a heat shroud80 having a rearwardly extending annular flange 82 spot welded as at 84to the annealed invar case 36. Spot welds 84 are preferably 12 in numberand equally spaced around the diameter. On the other hand, the heatshield retainer 70 is preferably spot welded to the diaphragm 64 by atotal of 16 welds spaced at equal angles at a 0.125 inch diameter.

FIGS. 3 and 4 are enlarged views of the end piece 50 of FIG. 2. This endpiece has an enlarged end 86 which, as shown in FIG. 2, engages sleeve40 and a smaller end 88 which projects through the correspondingaperture in the sleeve to be surrounded by the enlarged and inwardlyturned flange 52 of the sleeve. These two ends of the end piece 50 areseparated by a small groove 90. The end piece is preferably made ofannealed invar and has an overall diameter of 0.158 inch and an overallthickness of 0.030 inch.

FIGS. 5 and 6 show to an enlarged scale, the heat shield retainer 70 ofFIG. 1. retainer is likewise preferably made of annealed invar andcomprises a base 94 which is illustrated in FIG. 1 as spot welded to thediaphragm 64 and an annular flange 96 which is initially straight but isrolled by a suitable tool into the groove 74 of the heat shield as alsoillustrated in FIG. 1. The heat shield retainer 70 has an overalldiameter of 0. 155 inch in the embodiment described and a length fromthe base to the outer edge of the annular flange 96 of 0.056 inch.

FIGS. 7 and 8 are enlarged views of the heat shield 76 of FIG. 1. Theheat shield is provided with the previously described groove 74 and achamfered inner edge 98 chamfered at a 45 angle. The heat shield ispreferably formed of a ceramic identified as cordierite mix No. DC65EIl8, manufactured by the Du-Co Ceramics Company. It has an overalldiameter of 0.1460 inch and an overall length of 0.092 inch. Thediameter of the heat shield at the base of groove 74 is O. 136 inch.

FIGS. 9 and 10 are enlarged views of the heat shroud of FIG. 1. The heatshroud is preferably formed of annealed invar with an enlarged aperture102 in one end stepped to a smaller diameter at 104 and communicatingwith a smaller aperture 106 at its other end. The diameter of theaperture 106 is 0.163 inch, the diameter at 104 is 0.215 inch and thediameter at 102 is 0.238 inch. The overall diameter of the heat shroudis 0.249 inch and the total length is 0.200 inch. The length of aperture102 is 0.100 inch and the length of step 104 is 0.005 inch.

FIGS. 11 and 12 are enlarged views of the quartz crystal wafer package42 of FIG. 2. This package is in many respects similar to that disclosedin U.S. Pat. No. 3,349,259 and reference may be had to that patent for amore detailed description of its function and operation. Briefly, thepackage comprises a plurality of quartz crystal wafers, i.e., such asthe four crystal wafers 110, 112, 114, and 116 and a seismic mass 118illustrated in more detail in FIGS. 13 and 14.

Overlying some and spaced between others of the quartz crystals, areconductive electrodes 120, 122, 124, and 126, one of which is shown inmore detail in FIG. 15. The electrodes are preferably formed of goldhaving a 99.9 percent purity condition annealed and each include acentral aperture 128 and a pair of outwardly extending rectangular tabs130, which in the assembly of FIG. 11 are bent over and run along theside of the package to form electrical connections. The seismic mass isprovided with a counter bore 132 and is generally of cylindricalconfiguration but with four flattened edges 134, 136, 138, and 140Seismic mass 118 is preferably formed of heavy metal tungsten and has anoverall outer diameter of 0.148 inches with the distance betweenopposite flat surfaces being 0.136 inch. Active lead 22 passes throughthe aperture 128 in electrode 126 and through a corresponding aperturein quartz wafer 116 and is welded into the counterbore 132 of theseismic mass 118. The tabs 130 of electrodes 120, 124, and 126 arejoined together as at 139 in FIG. 1 1 as by welding so that whenassembled, both are in electrical contact with the conductive base 12 asillustrated in FIG. 2. The corresponding tabs 130 of electrode 122 arespaced and welded to the seismic mass 118 as at 141. The joined tabs ofthe electrodes 120, 124, and 126 are spaced from the seismic mass 118 bya flat rectangular sheet of insulation 142.

As more fully described in the aforementioned U.S. Pat. No. 3,349,259,the elements of the crystal package 42 are chosen so as to provideacceleration compensation and in this respect, the signal output ofquartz wafer 116 is opposite to and nullifies the output of one of theother three wafers 110, 112, and 114. To this end, the wafers arepositioned so that they develop charge at their opposite surfaces withthe polarity indicated in the drawing. The output signal is equivalentto the output of two of the wafers since the output of a third wafer iscancelled by the fourth. However, because of the intervening seismicmass 118, acting only on the quartz wafer 116, this wafer completelycancels any undesirable signals from all of the other three wafersresulting from acceleration forces. The

output signal is taken from the electrically conductive seismic mass 118by way of output lead 22 with the other side of the output (preferablygrounded) derived from electrode 126 by tion is directed primarily tothe elimination of temperature effects and other types of piezoelectriccrystal assemblies may be employed.

Referring specifically to FIG. 2, temperature compensation plate 44 isprovided for long-term temperature compensation for a reason more fullydescribed below. Buffer plate 46 is a rigid plate to permit welding ofthe preload sleeve 40 around its periphery and the coeflicient ofexpansion of buffer plate 46 is important for longterm temperaturecompensation as also described below. Cylindrical ceramic piece 48 is alow expansion, low mass, low conductivity ceramic material used to helpisolate the sensing element of the transducer from heat. End piece 50 isprovided to cooperate with the prestressed sleeve 40 used to provide aprestress load or squeezing force on the piezoelectric crystals, whichsleeve clamps and preloads the individual components to the base 12.

ln ordinary practice, the combined change in length with temperature ofthequartz package, temperature compensation plate, buffer plate, ceramicpiece, and end piece just described, would be adjusted to equal thechange in length of the preload sleeve 40 so that with a change intemperature, the net change in charge output of the piezoelectricpackage would be zero.,I-Iowever, this conventional arrangement has thesignificant disadvantage that for a rapid change in temperature appliedto the end piece 50 or to the preload sleeve 40, an output results sincethe preload sleeve expands, thereby changing the force applied to thequartz crystals, before the expansion of the other elements which aremore centrally located can offset the change.

In accordance with the present invention, the change in length is notmatched in the customary manner. The preload sleeve 40 and the end piece50 are instead fabricated from invar to provide a minimum of expansionin response to a rapid temperature variation and ceramic piece 48 ismade from cordierite which .has a low thermal expansion and also a lowthermal conductivity. In this manner, the sensing element of the presentinvention produces a minimum output from a rapid change in temperaturebut without the additional longterm compensation provided below, wouldotherwise have a significant output as the temperature change persists.

To avoid long-term temperature changes, a piezoelectric crystal havingtwomutually perpendicular sensitive axes is employed such as an X-cutquartz crystal. One of the axes referred to as the X-axis, coincideswith the longitudinal axis 150 of the transducer whereas the otherelectrically active axis referred to as the Y-axis is perpendicular tothe longitudinal axis 150 of the transducer. Because of the two activeaxes, any compression along the Y-axis will produce a charge outputsignal equal to but of opposite polarity to the charge generated bycompression along the X-axis.

Conventional practice for minimum charge output from changes intemperature is to provide a material in contact with the quartz surfacechosen to have a coefficient of expansion vvery near to that of thecoefficient along the Y-axis of the quartz. However, as previouslydescribed in the present invention, the materials of the preload sleeveand other elements are specifically chosen to provide minimum expansionrather than matched expansion, resulting in some output from stressesalong the X-axis due to long duration temperature change. This iscounteracted in the present invention by exerting a suitablecounteracting compression or tension on the transverse Y-axis.Obviously, if a number of suitable materials having differentcoefficients of expansion are available, one can be chosen for thetransverse expansion plate or temperature compensating plate 44.However, in actual practice, each individual transducer respondssomewhat differently, making the selection of materials difficult andtime consuming. A solution to the problem is to, in effect, provide thecompensation plate 44 with a variable range of temperature coefficientsof expansion. This is done by providing the buffer plate 46 from onematerial having a lower expansion coefficient than that of thepiezoelectric crystal and selecting the compensating plate 44 from amaterial having a higher coefficient (or vice versa). Then, by varyingthe thickness of plate 44, the ef fective coefficient of expansion ofthe surface in contact with the piezoelectric crystals may be made tovaryboth greateror less than that of the crystal thereby providingeither tension or compression to the crystal package which provideseither positive or negative output for long-term changes in temperature.

In the specific embodiment disclosed, the material of buffer plate 46has a thermal expansion coefiicient of 10 X 10" per degree centigradewhile the material of compensating plate 44 has a coefficient of 18 X10' per degree centigrade as contrasted with the Y-axis of quartzhavinga coefi'rcientof 14.3 X 10' per degree centigrade The material of thecasing 36 of FIG. 1 is also chosen to be invar so that changes intemperature will produce a minimum change in length and hence a minimumchange in force applied through the diaphragm.

Thus, it can be seen that the transducer of the present invention isdesigned in all respects to minimize variations in output due to shortterm temperature changes by eliminating or minimizing stresses caused byshort-term temperature variations. However, as a result, some long-termstresses along the X-axis may occur but any resulting long-termtemperature variations are separately taken care of or compensated forby the stresses imparted to the transverse or Y-axis of the quartzcrystals by the materials of the elements adjacent thereto. To furtherreduce short-term temperature variations, a substantially thin, flatdiaphragm 64 is employed. That is, if a flash of temperature is appliedto a flat sheet, then the surface nearest the flash will tend to expand.The surface away from the flash will not instantaneously changetemperature and will not expand. The result is that the sheet will tendto bend away from the heat until both surfaces reach the sametemperature. With an ordinary diaphragm, this warping results in a forceapplied to the piezoelectric column and hence an output due totemperature. This force is minimized in the present invention by makingthe diaphragm very thin and from a low thermal expansion material suchas invar to decrease the temperature difference stresses on oppositesides.

Although the diaphragm 64is fabricated from invar, when very hightemperatures are applied, the coefficient of expansion of invarincreases from 0.5 X 10 to 10 X 10' per degree centigrade. Under theseconditions, appreciable forces may be generated by the diaphragm andprotection is required. For internal combustion engines, a secondproblem is also encountered, i.e., the deposit of carbon. The solutionsto these problems afforded by the device of the present invention is theprovision of a shield for the diaphragm to shield it from directconduction and this is provided 'by the low conductivity, low expansion,ceramic piece or heat shield 76. Since some pressure must be applied tothe diaphragm, the shield does not completely block it from the pressuregases but rather is spaced by an annular gap from heat shroud 80. Withthis spacing, the pressure media in contact with the diaphragm is cooledsince the annular gap is made sufficiently small to contain a smallvolume. Theoretically, the annular gap should be tapered with as small avolume at the bottom as possible so that hot gases are not compressedinto the gap as far as they would be if a large volume was present atthe bottom of the annular groove. The width of the groove should besmall compared to the depth of the groove to provide maximum cooling. Ifdesired, the outer walls of the groove can be cooled for best heatresults. However, if the width of the groove is made too small, i.e., afew thousandths of an inch, then during use in an internal combustionengine, carbon may build up and bridge the gap resulting in a change ofsensitivity, linearity and hysteresis of the transducer. To discouragebuild-up of carbon, the heat shield 76 is madefrom a low heatconductivity ceramic. The outer surface of the shield may reach red hotor white hot, thereby burning away carbon deposits while maintainingminimum heat transfer to the diaphragm. The. heat shield material alsohas low thermal expansion thereby minimizing susceptibility to crackingwhen exposed to thermal shocks.

The groove illustrated in FIG. 1 is not tapered but is in effect steppeddue to the diameter of heat shield retainer 70 and this steppedconfiguration is a suitable approximation to a tapered groove. With theheat protection arrangement shown, a reduction in output from atransient temperature of 750:1 has been achieved over an unprotecteddiaphragm of similar construction.

It is apparent from the above that the present invention provides animproved piezoelectric transducer highly insensitive to temperaturechanges and one particularly suited for use with an engine gauge formeasuring pressures in internal combustion engines. The transducerprovides short-term temperature immunity and long-term temperaturecompensation so that it is able to measure pressures quite accuratelyeven during the scavenging and intake portions of the stroke as well asthe pressures immediately preceding the scavenging and intake portions.Thus, the transducer of the present invention is able to measurepressures quite accurately on the order of 10 psi even when as withinternal combustion engines, these pressures occur within a fewmilliseconds after pressures also accurately sensed by the transducer of1,000 psi or higher which high pressures are accompanied with flashtemperatures of 3,000 F. or more. The device of the present inventionprovides a 10-fold increase in accuracy over known transducers usablefor measuring pressures in internal combustion engines.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States LettersPatent is:

1. A pressure transducer comprising a piezoelectric sensing element, apressure diaphragm mechanically coupled to said sensing element forstressing said piezoelectric element in response to fluid pressureacting on said diaphragm, a heat shield mounted on said diaphragm on theside remote from said sensing element for protecting said diaphragm fromheat, said heat shield having a low coefficient of thermal expansion anda low thennal conductivity, a heat shroud surrounding but spaced fromsaid heat shield, said shroud and heat shield defining an annular groovecommunicating with said diaphragm, said groove being stepped to asmaller dimension at its end adjacent said diaphragm, said sensingelement having first and second mutually perpendicular electricallysensitive axes, said pressure diaphragm being mechanically coupled tostress said element along said first axis, and temperature compensatingmeans mechanically coupled to said element for applying compensatingstresses to said element along said second axis in response tovariations in temperature, said temperature compensating meanscomprising a pair of sideby-side masses positioned along said firstaxis, one of said masses engaging one side of said element, thetemperature coefficient of thermal expansion of one of said masses beinghigher and the other lower than the coefficient of thermal expansion ofsaid element along said second axis.

2. A pressure transducer comprising a piezoelectric sensing element, apressure diaphragm mechanically coupled to said sensing element forstressing said piezoelectric element in response to fluid pressureacting on said diaphragm, a heat shield mounted on said diaphragm on theside remote from said sensing element for protecting said diaphragm fromheat,

said heat shield having a low coefficient of thermal expansion and a lowthermal conductivity, a heat shroud surrounding but spaced from saidheat shield, said shroud and heat shield defining an annular groovecommunicating with said diaphragm, said sensing element having first andsecond electrically sensitive axes, said pressure diaphragm beingmechanically coupled'to stress said element along said first axis, andtemperature compensating means mechanically coupled to said element forapplying compensating stresses to said element along said second axis inresponse to variations in temperature, said groove being stepped to asmaller dimension at its end adjacent said diaphragm.

1. A pressure transducer comprising a piezoelectric sensing element, apressure diaphragm mechanically coupled to said sensing element forstressing said piezoelectric element in response to fluid pressureacting on said diaphragm, a heat shield mounted on said diaphragm on theside remote from said sensing element for protecting said diaphragm fromheat, said heat shield having a low coefficient of thermal expansion anda low thermal conductivity, a heat shroud surrounding but spaced fromsaid heat shield, said shroud and heat shield defining an annular groovecommunicating with said diaphragm, said groove being stepped to asmaller dimension at its end adjacent said diaphragm, said sensingelement having first and second mutually perpendicular electricallysensitive axes, said pressure diaphragm being mechanically coupled tostress said element along said first axis, and temperature compensatingmeans mechanically coupled to said element for applying compensatingstresses to said element along said second axis in response tovariations in temperature, said temperature compensating meanscomprising a pair of side-by-side masses positioned along said firstaxis, one of said masses engaging one side of said element, thetemperature coefficient of thermal expansion of one of said masses beinghigher and the other lower than the coefficient of thermal expansion ofsaid element along said second axis.
 2. A pressure transducer comprisinga piezoelectric sensing element, a pressure diaphragm mechanicallycoupled to said sensing element for stressing said piezoelectric elementin response to fluid pressure acting on said diaphragm, a heat shieldmounted on said diaphragm on the side remote from said sensing elementfor protecting said diaphragm from heat, said heat shield having a lowcoefficient of thermal expansion and a low thermal conductivity, a heatshroud surrounding but spaced from said heat shield, said shroud andheat shield defining an annular groove communicating with saiddiaphragm, said sensing element having first and second electricallysensitive axes, said pressure diaphragm being mechanically coupled tostress said element along said first axis, and temperature compensatingmeans mechanically coupled to said element for applying compensatingstresses to said element along said second axis in response tovariations in temperature, said groove being stepped to a smallerdimension at its end adjacent said diaphragm.